1. Field of the Invention
The present invention relates to a method of cooling a blade and to a blade with an airfoil profile for a gas turbine, comprising at least two opposite walls enclosing the inner part of the blade comprising cooling channels. The airfoil profile is extending from a bottom to a top part of the blade and at least one direct cooling fluid inlet is arranged at the bottom part of the blade.
2. Description of the Related Art
Gas turbine blades with an airfoil profile are used to drive the rotation of a rotor shaft in a gas turbine. The blades are fixed to the shaft along a circumference next to each other and along a rotational axis in parallel planes, with planes being perpendicular to the rotor axis. An airfoil profile of the blade extends from a bottom to a top part of the blade, where the bottom part is the part that is fixed to the shaft. The blades are cooled, for example, by air as the cooling fluid. The cooling fluid flows through cooling channels within the blade, removing heat from the blade, particularly by transporting the heat transferred from the blade to and stored in the cooling fluid to the outside of the turbine.
Blades, which are also called vanes, are produced from two pieces, which are joined together to form a blade. Within the blade on every piece a set of ribs is located. The ribs of the two pieces are in parallel and the pieces are joined together congruent, giving channels by joining together the ribs of the opposite pieces. The ribs are arranged in parallel at every piece and the pieces are of a structure of opposite hand. The resulting cooling channels, formed in-between the ribs inside the blade, are mainly arranged parallel to the rotating axis with an inlet for cooling fluid on one side, a sucking side of the airfoil profile and an outlet at the other side of the profile. There is no direct feeding of cooling fluid at the bottom part of the blade.
The bottom part of the blade, especially at the trailing edge area of the airfoil, is very critical in terms of its thermal state and stress. An increase of cooling effectiveness in this area of the blade requires an increase of the cooling fluid mass flow. An increase in cooling fluid mass flow results in a drop of turbine efficiency.
EP 1895096 A1 discloses a way to improve the cooling effectiveness in the bottom part of the blade, which comprises a direct cooling fluid feeding for that part of the airfoil from a blade inlet in the bottom part. This can result in a sufficient cooling effectiveness of the bottom part. The design of cooling channels differs to the before described design, for example, by cooling channels not in parallel anymore to the axis of the rotator. With ribs on a piece arranged with equal distance to the neighboring ribs, all cooling channels have respectively the same width, i.e., cross section d. The cross section d is calculated according to a considerable hydraulic resistance for the cooling fluid and heat transfer. A direct cooling fluid feeding for the airfoil from a blade inlet in the bottom part exhibits in general a smaller hydraulic resistance and heat transfer from the blade to the cooling fluid. This can result in an outlet area of the ribs set which is too large, resulting in a too large cooling fluid mass flow, with disadvantages as described before. A solution is to place an orifice at the blade inlet in the bottom part to prevent too large values of mass flow of the cooling fluid in the bottom area of the blade. The orifice introduces an extra hydraulic resistance and pressure drop at the orifice, decreasing the cooling effectiveness, as compared with a maximal possible without orifice. For a sufficient level of cooling effectiveness in the bottom part, an additional cooling fluid mass flow is necessary. This results in a drop in the effectiveness of the turbine.